Wind turbines typically comprise a nacelle arranged on top of a tower of the wind turbine. A main shaft is stationary arranged within the nacelle that rotatably mounts a rotor hub with a plurality of rotor blades via a main bearing. The main bearing may be arranged as a tapered bearing capable of supporting axial and radial loads. Typically, the main bearing is arranged between an end of the main shaft facing the rotor hub and mounting means arranged on an outer surface of the rotor hub, in particular flanges.
Arrangements for rotatably mounting the rotor hub to the main shaft utilising more than one bearing are also known. WO 2011/071378 A2 describes a wind turbine with a rotor hub firmly connected to a tube member that has two tapered roller bearings for rotatably mounting the rotor hub to the main shaft. The two tapered roller bearings are located relatively close to each other, so that both bearings carry a part of the total weight of rotating parts of the wind turbine.
WO 2011/076795 A2 describes a rotor hub for a wind turbine that is made from two or more moulded hub parts that form, when assembled, a hollow shell. Each hub part comprises at least a section of the blade bearing flange adapted to be connected to one of the rotor blades of the wind turbines. Furthermore, each hub part comprises at least a connecting portion that is either adapted to be connected to a corresponding connecting portion of another hub part or to be attached to a stiffening element that provides the hub part with additional stability. In the latter case, the stiffening element extends radial inwards into an interior defined by the hollow shell when assembled. The rotor hub is rotably mounted to the main shaft via a bearing that is attached to a main shaft flange arranged on an outer surface of the hollow shell.
Wind turbines may be directly or indirectly driven. Directly driven wind turbines transfer the torque generated by wind passing over the rotor blades directly to a rotor of a generator that is rotatably arranged with respect to a stator of a wind turbine. Electric power is generated by magnetic induction. Generators capable of converting mechanical energy into electrical power are well established in the state of art. Indirectly driven wind turbines comprise a gear box or a transmission arranged between the rotating rotor hub and the rotor of the generator allowing for an adjustment of the angular velocity of the rotor with respect to the stator.
With the recent development of constructing larger wind turbines capable of generating electrical power of more than 6 MW, rotor hubs and rotor blades have significantly increased in size. Consequently, the bearing rotatably mounting the rotor hub has to support bigger loads. Even today, the bearing is a major cost driver when constructing wind turbines.